Provide a consistent way for MCP Apps to know which tool invoked them

[billyjanitsch]

Is your feature request related to a problem? Please describe.
If multiple app tools are linked to the same UI resource, the rendered app may wish to know which tool was called to invoke it, e.g., to decide how to parse tool inputs/results.

The spec allows for the host to pass toolInfo in HostContext via ui/initialize, but it's an optional parameter. Tool info is not sent in any other host -> app message.

I'm curious why this parameter is optional. It seems like it ought to always be available on the host, and it seems reasonable/useful for an app to require it.

A current workaround is for the MCP server to manually include the tool name in structuredContent of the tool response, though this solution requires the app to wait until ui/notifications/tool-result even though the tool name is known by the host and server much earlier.

Describe the solution you'd like
Make toolInfo required in HostContext via ui/initialize.

Describe alternatives you've considered
The host could also send the tool name in ui/notifications/tool-input/ui/notifications/tool-input-partial.

Additional context
None.

[jerelmiller]

I believe this is already covered with host context via toolInfo.name. That said, that type is optional so some hosts might not set it (I'm seeing this with Goose right now for example).

[billyjanitsch]

@jerelmiller Thanks, I overlooked that! Updated the issue body accordingly.

It's nice that toolInfo exists but it's unfortunate that it's optional, because it means that a workaround is still necessary if you want to build an app that requires it. I wonder if there's a particular reason that it was made optional.

[jerelmiller]

Yes I'm wondering the same thing. I'm building a tool that integrates with MCP apps and had to merge a fix today that does exactly this (apollographql/apollo-client-ai-apps#63). I'd love to understand the optionality here and/or at least get some guidance on a "best practice" in case a host decides not to set toolInfo.